With the advent of high speed inkjet printing, dry times of printed images have become an important issue because the inkjet inks must dry quickly in order to prevent smearing of the printed image. To provide fast dry times, microporous inkjet print media have been developed. The microporous inkjet print media use particles or pigments having a large surface area that are held together with a small amount of binder. However, the microporous inkjet print media have poor cohesive strength, which leads to poor scratch resistance. The pores in the microporous inkjet print media also allow substantial airfade of the printed images due to the large surface area that is exposed to air.
In order to obtain printed images that dry quickly and have good image quality, durability, and permanence, microporous inkjet print media with thermally laminated barrier layers have been developed. While lamination of the printed image provides very good image quality and permanence, the cost of producing the laminated images is increased due to the cost of the laminator and the additional supplies that are necessary.
In addition to lamination techniques, the permanence and print quality of printed images have been improved by using a sealable or fusible layer on the print medium. After the desired image is printed, the print medium is exposed to heat and/or pressure to seal the sealable layer over the printed image. The sealable layer forms a film over the printed image, helping to protect the image from scratches or fading. With sealable print medium, the print quality of the image is low until the sealable layer is heated. In other words, photographic resolution is only achieved after the print medium is sealed. Sealable layers are typically formed from a single, small particle size polymeric material having either a low glass transition temperature (“Tg”) or a high Tg. Although these polymeric materials are capable of producing clear films, they are problematic because they shrink when exposed to stresses, unless a large amount of binder is added. However, increasing the amount of binder in the sealable layer compromises the fastness properties. In order to reduce shrinkage, a single, large particle size polymeric material having a high Tg is frequently used. However, using this polymeric material increases the temperature to which the print medium must be exposed to seal the print medium. A heat-sealable print medium that uses a single, large particle size latex having a high Tg can be sealed using a laminator or heated calender type device. The image produced on this print medium, although of acceptable quality after sealing, has shown significant preprint durability issues, which leads to fading in any damaged areas.
Water resistant coatings for use on inkjet printing substrates have also been disclosed. The water resistant coating typically includes plastic pigments of various sizes. Disclosed plastic pigments include styrene polymers, acrylic polymers, or acrylic urethane polymers having particles sizes between 100 and 1500 nm. The water resistant coating provides lightfastness and scratch resistance to printing substrates that are used indoors or outdoors.
Use of a recording medium having a porous outer layer has also been disclosed. The porous outer layer is formed from a thermoplastic resin having a particle diameter of 0.1 μm to 5 μm. By exposing the porous outer layer to heat, the porous outer layer is melted to form a nonporous layer. Images printed on the recording medium have high image density and are resistant to weather.
What is desired is a print medium that is scratch resistant and resistant to airfade. In addition, the print medium should provide good print quality both before and after sealing of the heat-sealable layer so that a user may choose whether to seal the print medium.